The invention relates generally to detectable labels and compositions useful in assay methods for detecting soluble, suspended, or particulate substances or analytes such as proteins, carbohydrates, nucleic acids, bacteria, viruses, and eukaryotic cells and more specifically relates to compositions and methods that include luminescent (phosphorescent or fluorescent) labels.
Methods for detecting specific macromolecular species, such as proteins, drugs, and polynucleotides, have proven to be very valuable analytical techniques in biology and medicine, particularly for characterizing the molecular composition of normal and abnormal tissue samples and genetic material. Many different types of such detection methods are widely used in biomedical research and clinical laboratory medicine. Examples of such detection methods include: immunoassays, immunochemical staining for microscopy, fluorescence-activated cell sorting (FACS), nucleic acid hybridization, water sampling, air sampling, and others.
Typically, a detection method employs at least one analytical reagent that binds to a specific target macromolecular species and produces a detectable signal. These analytical reagents typically have two components: (1) a probe macromolecule, for example, an antibody or oligonucleotide, that can bind a target macromolecule with a high degree of specificity and affinity, and (2) a detectable label, such as a radioisotope or covalently-linked fluorescent dye molecule. In general, the binding properties of the probe macromolecule define the specificity of the detection method, and the detectability of the associated label determines the sensitivity of the detection method. The sensitivity of detection is in turn related to both the type of label employed and the quality and type of equipment available to detect it.
For example, radioimmunoassays (RIA) have been among the most sensitive and specific analytical methods used for detecting and quantitating biological macromolecules. Radioimmunoassay techniques have been used to detect and measure minute quantities of specific analytes, such as polypeptides, drugs, steroid hormones, polynucleotides, metabolites, and tumor markers, in biological samples. Radioimmunoassay methods employ immunoglobulins labeled with one or more radioisotopes as the analytical reagent. Radiation (xcex1, xcex2, or xcex3) produced by decay of the attached radioisotope label serves as the signal which can be detected and quantitated by various radiometric methods.
Radioisotopic labels possess several advantages, such as: very high sensitivity of detection, very low background signal, and accurate measurement with precision radiometric instruments (scintillation and gamma counters) or with inexpensive and sensitive autoradiographic techniques. However, radioisotopic labels also have several disadvantages, such as: potential health hazards, difficulty in disposal, special licensing requirements, and instability (radioactive decay and radiolysis). Further, the fact that radioisotopic labels typically do not produce a strong (i.e., non-Cerenkov) signal in the ultraviolet, infrared, or visible portions of the electromagnetic spectrum makes radioisotopes generally unsuitable as labels for applications, such as microscopy, image spectroscopy, and flow cytometry, that employ optical methods for detection.
For these and other reasons, the fields of clinical chemistry, water and air monitoring, and biomedical research have sought alternative detectable labels that do not require radioisotopes. Examples of such non-radioactive labels include: (1) enzymes that catalyze conversion of a chromogenic substrate to an insoluble, colored product (e.g., alkaline phosphatase, xcex2-galactosidase, horseradish peroxidase) or catalyze a reaction that yields a fluorescent or luminescent product (e.g., luciferase) (Beck and Koster (1990) Anal. Chem. 62: 2258; Durrant, I. (1990) Nature 346: 297; Analytical Applications of Bioluminescence and Chemiluminescence (1984) Kricka et al. (Eds.) Academic Press, London), and (2) direct fluorescent labels (e.g., fluorescein isothiocyanate, rhodamine, Cascade blue), which absorb electromagnetic energy in a particular absorption wavelength spectrum and subsequently emit visible light at one or more longer (i.e., less energetic) wavelengths.
Using enzymes and phosphorescent/fluorescent or calorimetric detectable labels offers the significant advantage of signal amplification, since a single enzyme molecule typically has a persistent capacity to catalyze the transformation of a chromogenic substrate into detectable product. With appropriate reaction conditions and incubation time, a single enzyme molecule can produce a large amount of product, and hence yield considerable signal amplification. However, detection methods that employ enzymes as labels disadvantageously require additional procedures and reagents in order to provide a proper concentration of substrate under conditions suitable for the production and detection of the colored product. Further, detection methods that rely on enzyme labels typically require prolonged time intervals for generating detectable quantities of product, and also generate an insoluble product that is not attached to the probe molecule.
An additional disadvantage of enzyme labels is the difficulty of detecting multiple target species with enzyme-labeled probes. It is problematic to optimize reaction conditions and development time(s) for two or more discrete enzyme label species and, moreover, there is often considerable spectral overlap in the chromophore endproducts which makes discrimination of the reaction products difficult.
Fluorescent labels do not offer the signal amplification advantage of enzyme labels, nonetheless, fluorescent labels possess significant advantages which have resulted in their widespread adoption in immunocytochemistry. Fluorescent labels typically are small organic dye molecules, such as fluorescein, Texas Red, or rhodamine, which can be readily conjugated to probe molecules, such as immunoglobulins or Staph. aureus Protein A. The fluorescent molecules (fluorophores) can be detected by illumination with light of an appropriate excitation frequency and the resultant spectral emissions can be detected by electro-optical sensors or light microscopy.
A wide variety of fluorescent dyes are available and offer a selection of excitation and emission spectra. It is possible to select fluorophores having emission spectra that are sufficiently different so as to permit multitarget detection and discrimination with multiple probes, wherein each probe species is linked to a different fluorophore. Because the spectra of fluorophores can be discriminated on the basis of both narrow band excitation and selective detection of emission spectra, two or more distinct target species can be detected and resolved (Titus et al. (1982) J. Immunol. Methods 50: 193; Nederlof et al. (1989) Cytometry 10: 20; Ploem, J. S. (1971) Ann. NY Acad. Sci. 177: 414).
Unfortunately, detection methods which employ fluorescent labels are of limited sensitivity for a variety of reasons. First, with conventional fluorophores it is difficult to discriminate specific fluorescent signals from nonspecific background signals. Most common fluorophores are aromatic organic molecules which have broad absorption and emission spectra, with the emission maximum red-shifted 50-100 nm to a longer wavelength than the excitation (i.e., absorption) wavelength. Typically, both the absorption and emission bands are located in the UV/visible portion of the spectrum. Further, the lifetime of the fluorescence emission is usually short, on the order of 1 to 100 ns. Unfortunately, these general characteristics of organic dye fluorescence are also applicable to background signals which are contributed by other reagents (e.g., fixative or serum), or autofluorescence or the sample itself (Jongkind et al. (1982) Exp. Cell Res. 138: 409; Aubin, J. E. (1979) J. Histochem. Cytochem. 27: 36). Autofluorescence of optical lenses and reflected excitation light are additional sources of background noise in the visible spectrum (Beverloo et al. (1991) Cytometry 11: 784; Beverloo et al. (1992) Cytometry 13: 561). Therefore, the limit of detection of specific fluorescent signal from typical fluorophores is limited by the significant background noise contributed by nonspecific fluorescence and reflected excitation light.
A second problem of organic dye fluorophores that limits sensitivity is photolytic decomposition of the dye molecule (i.e., photobleaching). Thus, even in situations where background noise is relatively low, it is often not possible to integrate a weak fluorescent signal over a long detection time, since the dye molecules decompose as a function of incident irradiation in the UV and near-UV bands.
However, because fluorescent labels are attractive for various applications, several alternative fluorophores having advantageous properties for sensitive detection have been proposed. One approach has been to employ organic dyes comprising a phycobiliprotein acceptor molecule dye that emits in the far red or near infrared region of the spectrum where nonspecific fluorescent noise is reduced. Phycobiliproteins are used in conjunction with accessory molecules that effect a large Stokes shift via energy transfer mechanisms (U.S. Pat. No. 4,666,862; Oi et al. (1982) J. Cell. Biol. 93: 891). Phycobiliprotein labels reduce the degree of spectral overlap between excitation frequencies and emission frequencies. An alternative approach has been to use cyanine dyes which absorb in the yellow or red region and emit in the red or far red where autofluorescence is reduced (Mujumbar et al. (1989) Cytometry 10: 11).
However, with both the phycobiliproteins and the cyanine dyes the emission frequencies are red-shifted (i.e., frequency downshifted) and emission lifetimes are short, therefore background autofluorescence is not completely eliminated as a noise source. More importantly perhaps, phycobiliproteins and cyanine dyes possess several distinct disadvantages: (1) emission in the red, far red, and near infrared region is not well-suited for detection by the human eye, hampering the use of phycobiliprotein and cyanine labels in optical fluorescence microscopy, (2) cyanines, phycobiliproteins, and the coupled accessory molecules (e.g., Azure A) are organic molecules susceptible to photobleaching and undergoing undesirable chemical interactions with other reagents, and (3) emitted radiation is down-converted, i.e., of longer wavelength(s) than the absorbed excitation radiation. For example, Azure A absorbs at 632 nm and emits at 645 nm, and allophycocyanin absorbs at 645 nm and emits at 655 nm, and therefore autofluorescence and background noise from scattered excitation light is not eliminated.
Another alternative class of fluorophore that has been proposed are the down-converting luminescent lanthanide chelates (Soini and Lovgren (1987) CRC Crit. Rev. Anal. Chem. 18: 105; Leif et al. (1977) Clin. Chem. 23: 1492; Soini and Hemmila (1979) Clin. Chem. 25: 353; Seveus et al. (1992) Cytometry 13: 329). Down-converting lanthanide chelates are inorganic phosphors which possess a large downward Stokes shift (i.e., emission maxima is typically at least 100 nm greater than absorption maxima) which aids in the discrimination of signal from scattered excitation light. Lanthanide phosphors possess emission lifetimes that are sufficiently long (i.e., greater than 1 xcexcs) to permit their use in time-gated detection methods which can reduce, but not totally eliminate, noise caused by shorter-lived autofluorescence and scattered excitation light. Further, lanthanide phosphors possess narrow-band emission, which facilitates wavelength discrimination against background noise and scattered excitation light, particularly when a laser excitation source is utilized (Reichstein et al. (1988) Anal. Chem. 60: 1069). Recently, enzyme-amplified lanthanide luminescence using down-converting lanthanide chelates has been proposed as a fluorescent labeling technique (Evangelista et al. (1991) Anal. Biochem. 197: 213; Gudgin-Templeton et al. (1991) Clin Chem. 37: 1506).
Until recently, down-converting lanthanide phosphors have had the significant disadvantage that their quantum efficiency in aqueous (oxygenated) solutions is so low as to render them unsuitable for cytochemical staining. Beverloo et al. (op.cit.) have described a particular down-converting lanthanide phosphor (yttrium oxysulfide activated with europium) that produces a signal in aqueous solutions which can be detected by time-resolved methods. Seveus et al. (op.cit.) have used down-converting europium chelates in conjunction with time-resolved fluorescence microscopy to reject the signal from prompt fluorescence and thereby reduce autofluorescence. Tanke et al. (U.S. Pat. No. 5,043,265) report down-converting phosphor particles as labels for immunoglobulins and polynucleotides.
However, the down-converting lanthanide phosphor of Beverloo et al. and the europium chelate of Seveus et al. require excitation wavelength maxima that are in the ultraviolet range, and thus produce significant sample autofluorescence and background noise (e.g., serum and/or fixative fluorescence, excitation light scattering and refraction, etc.) that must be rejected (e.g., by filters or time-gated signal rejection). Further, excitation with ultraviolet irradiation damages nucleic acids and other biological macromolecules, posing serious problems for immunocytochemical applications where it is desirable to preserve the viability of living cells and retain cellular structures (e.g., FACS, cyto-architectural microscopy).
Laser scanning fluorescence microscopy has been used for two-photon excitation of a UV-excitable fluorescent organic dye, Hoechst 33258, using a stream of strongly focused laser pulses (Denk et al. (1990) Science 248: 73). The organic fluorphore used by Denk et al. was significantly photobleached by the intense, highly focused laser light during the course of imaging. Motsenbocker et al. (EP 476 556) describes a method to increase luminol chemiluminescence by adding a dye catalyst that absorbs long wavelength radiation (deep red light) and subsequently reacts with molecular oxygen to generate an oxidant which can itself react with luminol and produce oxidized luminol which emits blue light. Gavrilovic (U.S. Pat. No. 5,166,948) discloses a method and apparatus for optical pumping of infrared pump light to a visible or ultraviolet emiision light having a wavelength shorter than the pump light (i.e., up-converted emisison). Xu et al. (1995) J. Phys. Chem. 99: 4447reports up-conversion emission from Er+3-doped sol-gel silica glasses.
Thus, there exists a significant need in the art for labels and detection methods that permit sensitive optical and/or spectroscopic detection of specific label signal(s) with essentially total rejection of nonspecific background noise, and which are compatible with intact viable cells and aqueous or airborne environments.
The references discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention.
The present invention provides labels, detection methods, and detection apparatus which permit ultrasensitive detection of cells, biological macromolecules, and other analytes, which can be used for multiple target detection and target discrimination. The up-converting labels of the invention permit essentially total rejection of non-specific background autofluorescence and are characterized by excitation and emitted wavelengths that are typically in the infrared or visible portions of the spectrum, respectively, and thus avoid the potentially damaging effects of ultraviolet radiation. The up-converting labels of the invention convert long-wavelength excitation radiation (e.g., near-IR) to emitted radiation at about one-half to one-third the wavelength of the excitation wavelength. Since background fluorescence in the visible range is negligible if near-IR excitation wavelengths are used, the use of up-converting labels provides essentially background-free detection of signal.
In brief, the invention provides the use of luminescent materials that are capable of multiphoton excitation and have upshifted emission spectra. In one embodiment of the invention, up-converting phosphors (i.e., which absorb multiple photons in a low frequency band and emit in a higher frequency band) are used as labels which can be linked to one or more probes, such as an immunoglobulin, polynucleotide, streptavidin, Protein A, receptor ligand, or other probe molecule. In an another embodiment, up-converting organic dyes serve as the label. The organic dye labels and phosphor labels of the invention are highly compatible with automated diagnostic testing, microscopic imaging applications, and coded particle detection, among many other applications.
The nature of the invention provides considerable flexibility in the apparatus for carrying out the methods. As a general matter, the excitation source may be any convenient light source, including inexpensive near-infrared laser diodes or light-emitting diodes (LEDs), and the detector may be any convenient detector, such as a photodiode. In the case of a single reporter, the apparatus includes a laser diode capable of emitting light at one or more wavelengths in the reporter""s excitation band and a detector that is sensitive to at least some wavelengths in the reporter""s emission band. The laser light is preferably focused to a small region in the sample, and light emanating from that region is collected and directed to the detector. An electrical signal representing the intensity of light in the emission band provides a measure of the amount of reporter present. Depending on the detector""s spectral response, it may be necessary to provide a filter to block the excitation light.
Simultaneous detection of multiple reporters is possible, at least where the reporters have different excitation bands or different emission bands. Where the excitation bands differ, multiple laser diodes emitting at respective appropriate wavelengths are combined using a wavelength division multiplexer or other suitable techniques, such as frequency labeling, frequency modulation, and lock-in detector device. If the emission bands are different (whether or not the excitation bands are different), light in the different emission bands is separated and sent to multiple detectors. If the emission bands overlap, a single detector may be used, but other detection techniques are used. One example is to use time multiplexing techniques so that only one reporter is emitting at a given time. Alternatively, the different laser diodes can be modulated at different characteristic frequencies and lock-in detection performed.
Detection methods and detection apparatus of the present invention enable the ultrasensitive detection of up-converting phosphors and up-converting organic dyes by exploiting what is essentially the total absence of background noise (e.g., autofluorescence, serum/fixative fluorescence, excitation light scatter) that are advantageous characteristics of up-converting labels. Some embodiments of the invention utilize time-gated detection and/or wavelength-gated detection for optimizing detection sensitivity, discriminating multiple samples, and/or detecting multiple probes on a single sample. Phase-sensitive detection can also be used to provide discrimination between signal(s) attributable to an up-converting phosphor and background noise (e.g. autofluorescence) which has a different phase shift.
Up-converting organic dyes, such as red-absorbing dyes, also can be used in an alternate embodiment that converts the photons absorbed by the dye into a transient voltage that can be measured using electrodes and conventional electronic circuitry. After having undergone two-photon absorption the dye is ionized by additional photons from the light source (e.g., a laser) leading to short-lived molecular ions whose presence can be detected and quantified by measuring the transient photoconductivity following the excitation irradiation. In this embodiment, resonant multiphoton ionization is used to provide a quantitative measurement of the number and/or concentration of dye molecules in a sample. Furthermore, essentially all photoions formed in the irradiated sample contribute to the signal, whereas photons are emitted isotropically and only a fraction can be collected using optics. Measurement of the transient photocurrent effectively transfers the conversion of photons into an electronic signal that is readily measured with relatively simple and inexpensive sensors such as electrodes.
In some embodiments, the present invention utilizes one or more optical laser sources for generating excitation illumination of one or more discrete frequency(ies). In certain variations of the invention, laser irradiation of an up-converting label can modify the immediate molecular environment through laser-induced photochemical processes involving either direct absorption or energy transfer; such spatially-controlled deposition of energy can be used to produce localized damage and/or to probe the chemical environment of a defined location. In such embodiments, the up-converting label can preferably act as a photophysical catalyst.
The invention provides methods for producing targeted damage (e.g., catalysis) in chemical or biological materials, wherein a probe is employed to localize a linked up-converting label to a position near a targeted biological structure that is bound by the probe. The localized up-converting label is excited by one or more excitation wavelengths and emit at a shorter wavelength which may be directly cytotoxic or genotoxic (e.g., by producing free radicals such as superoxide, and/or by generating thymine-thymine dimers), or which may induce a local photolytic chemical reaction to produce reactive chemical species in the immediate vicinity of the label, and hence in the vicinity of the targeted biological material. Thus, targeting probes labeled with one or more up-converting labels (e.g., an up-converting inorganic phosphor) may be used to produce targeted damage to biological structures, such as cells, tissues, neoplasms, vasculature, or other anatomical or histological structures.
Embodiments of the present invention also include up-converting phosphors which can also be excited by an electron beam or other beam of energetic radiation of sufficient energy and are cathodoluminescent. Such electron-stimulated labels afford novel advantages in eliminating background in ultrasensitive biomolecule detection methods. Typically, stimulation of the up-converting phosphor with at least two electrons is employed to generate a visible-light or UV band emission.
The invention also provides for the simultaneous detection of multiple target species by exploiting the multiphoton excitation and subsequent background-free fluorescence detection of several up-converting phosphors or up-converting dyes. In one embodiment, several phosphors/dyes are selected which have overlapping absorption bands which allow simultaneous excitation at one wavelength (or in a narrow bandwidth), but which vary in emission characteristics such that each probe-label species is endowed with a distinguishable fluorescent xe2x80x9cfingerprint.xe2x80x9d By using various methods and devices, the presence and concentration of each of the phosphors or dyes can be determined.
The invention also provides biochemical assay methods for determining the presence and concentration of one or more analytes, typically in solution. The assay methods employ compositions of probes labeled with up-converting phosphors and/or up-converting dyes and apparatus for magnetically and/or optically trapping particles that comprise the analyte and the labeled probe. In one embodiment, a sandwich assay is performed, wherein an immobilized probe, immobilized on a particle, binds to a predetermined analyte, producing an immobilization of the bound analyte on the particle; a second probe, labeled with an up-converting label can then bind to the bound analyte to produce a bound sandwich complex containing an up-converting label bound to a particle. By combining different probe-label combinations, particles of various sizes, colors, and/or shapes with distinct immobilized probe(s), and/or various excitation wavelengths, it is possible to perform multiple assays essentially simultaneously or contemporaneously. This multiplex advantage affords detection and quantitation of multiple analyte species in a single sample. The assay methods are also useful for monitoring the progress of a reaction, such as a physical, chemical, biochemical, or immunological reaction, including binding reactions. For example, the invention may be used to monitor the progress of ligand-binding reactions, polynucleotide hybridization reactions, including hybridization kinetics and thermodynamic stability of hybridized polynucleotides.
The invention also provides methods, up-converting labels, and compositions of labeled binding reagents for performing fluorescence-activated cell sorting (FACS) by flow cytometry using excitation radiation that is in the infrared portion of the spectrum and does not significantly damage cells. This provides a significant advantage over present FACS methods which rely on excitation illumination in the ultraviolet portion of the spectrum, including wavelengths which are known to produce DNA lesions and damage cells.
The invention also provides compositions comprising at least one fluorescent organic dye molecule attached to an inorganic up-converting phosphor. The fluorescent organic dye molecule is selected from the group consisting of: rhodamines, cyanines, xanthenes, acridines, oxazines, porphyrins, and phthalocyanines, and may optionally be complexed with a heavy metal. The fluorescent organic dye may be adsorbed to the inorganic up-converting phosphor crystal and/or may be covalently attached to a coated inorganic up-converting phosphor, a derivatized vitroceramic up-converting phosphor, or a microencapsulated inorganic up-converting phosphor. Frequently, covalent conjugation between the up-converting inorganic phosphor particles and proteins (e.g., avidin, immunoglobulin) can be accomplished with heterobifunctional crosslinkers.